Stump to Pump: Sustainable Fuel Production from Biomass

Diminishing crude oil reserves and an increasing global population drive the worldwide search for clean and sustainable energy. Biofuels, derived from cellulose, offer a promising pathway towards the sustainable production of carbon-neutral transportation fuels. To efficiently generate biofuels from plant materials, we have adopted a comprehensive “stump to pump” approach that seeks to optimize and seamlessly integrate each step of the biofuel production process, from the planting and harvesting of energy crops to the chemical conversion of cellulose to fuels compatible with our existing infrastructure. The first step towards the successful production of biofuels involves the growth and handling of biomass feedstocks. Our research team investigates the viability of various types of biomass as process feedstocks, the efficient growth of biomass, and the logistics of transporting biomass to a conversion facility. In the next step of biofuels production, biomass is converted to syngas, a mixture of hydrogen and carbon monoxide, which can be reacted over a catalyst to produce liquid fuels. Using our institution’s demonstration-scale fluidized-bed gasifier, we can study the production of syngas from different feedstocks as well as the subsequent syngas purification processes. Finally, our investigative team’s expertise in syngas conversion pathways such as Fischer-Tropsch Synthesis allows us to develop reaction schemes for transforming our cellulose-derived syngas into fuels. By interlinking and optimizing each step, our “stump to pump” view of sustainable fuel generation brings the commercialization of biofuels closer to economic reality.

Stump to Pump: Sustainable Fuel Production from Biomass

Diminishing crude oil reserves and an increasing global population drive the worldwide search for clean and sustainable energy. Biofuels, derived from cellulose, offer a promising pathway towards the sustainable production of carbon-neutral transportation fuels. To efficiently generate biofuels from plant materials, we have adopted a comprehensive “stump to pump” approach that seeks to optimize and seamlessly integrate each step of the biofuel production process, from the planting and harvesting of energy crops to the chemical conversion of cellulose to fuels compatible with our existing infrastructure. The first step towards the successful production of biofuels involves the growth and handling of biomass feedstocks. Our research team investigates the viability of various types of biomass as process feedstocks, the efficient growth of biomass, and the logistics of transporting biomass to a conversion facility. In the next step of biofuels production, biomass is converted to syngas, a mixture of hydrogen and carbon monoxide, which can be reacted over a catalyst to produce liquid fuels. Using our institution’s demonstration-scale fluidized-bed gasifier, we can study the production of syngas from different feedstocks as well as the subsequent syngas purification processes. Finally, our investigative team’s expertise in syngas conversion pathways such as Fischer-Tropsch Synthesis allows us to develop reaction schemes for transforming our cellulose-derived syngas into fuels. By interlinking and optimizing each step, our “stump to pump” view of sustainable fuel generation brings the commercialization of biofuels closer to economic reality.

While the three of us are researching and monitoring carbon, particularly CO and CO2, in the conversion steps exclusively, we do have IGERT colleagues actively investigating related subjects such life cycle analysis. With respect to the overall stump-to-pump process being carbon neutral, it is possible only if each part of the process uses energy from a green source.

If we do assume that each of the logistical or processing steps such as harvesting, grinding, transport, and fuel refining is fueled exclusively using biomass or biofuels, the overall picture must be carbon neutral. In that case, the only carbon ‘in’ is CO2 uptake by the biomass (pine, switchgrass, etc.) and the only CO2 ‘out’ is via the CO2 produced in biomass gasification, wood burning for process heat, the FTS reaction, and the eventual combustion of the liquid fuel.

In terms of a mass balance, carbon in must be the same as carbon out, and as CO2 is the only carbon source as well as the ultimate carbon compound produced, the overall process must net to zero.

As an ardent environmentalist and tree-lover, I felt uncomfortable in your use of trees as the source of biomass. You don’t mention the sustainability aspects of tree use. I presume that the trees you fell are replaced, but the virgin forests you cut down may have a different distribution of tree species. How do you know that this will not affect the ecosystem that the tree-felling has destroyed? And surely you face a lot of public disapproval of this source of bioenergy. How will you deal with these societal aspects?

You certainly are not alone in that thought, and the societal aspects of this research are some of the most challenging!

You are correct in the assumption that the trees would be replaced. The sustainable nature of tree harvesting has been well studied and improved upon due to the pulp and paper industry. In your mention of negative environmental aspects, we can only assume that virgin forest development (post-harvest) would behave in a similar fashion to that of dedicated pulp and paper land.

The only way to persuade the general public that this is a sustainable option, and not a “Lorax” scenario would be through public education. The sad truth is that very few people would be willing to give up electricity and fuels/lubricants today, despite the methods in which they are generated. Currently, the vast majority of electrical/fuel/lubricant generation is sourced from oil, natural gas, and coal. In order to recover those natural resources, processes such as strip mining, fraking, and dead-sea drilling have been utilized. From an environmental standpoint, using a renewable source such as forested land and replanting that land, does far less damage. It is only through public education of current fuel and electricity generation techniques, that the public will accept tree biomass as an sustainable alternative for energy.

Thank you for the thoughtful and honest response. I agree with some of the points that you make. However, while strip mining is generally very destructive to the environment, “fraking” should not (necessarily- depending on the implementation) be put into the same category. It is a largely misunderstood energy source and it too suffers from societal disapproval that I mentioned for deforestation.
I agree that we all want the lights to turn on when we flick the light switch. It’s a conundrum that we all have to work to solve with the best interests of the planet firmly in mind.

To add, I think one point that merits some further clarification is the idea that the use of woody biomass would even result in a net deforestation, particularly of virgin forest. Currently, there is definitely more growth than drain on the total biomass in the southeast and it has been shown that sustainably producing a billion tons per year of biomass is feasible. (http://wp.auburn.edu/igert/wp-content/uploads/2...)

Additionally, managed forest land is on the order of 2-3 times more productive than non-managed land. Add in that planting pine is done in rotations so that all the trees are of approximately the same size (easier to harvest) and managed land that treats pine as just another energy crop becomes a far more attractive source of biomass than old growth virgin forest.

Finally, a significant portion of bioenergy demand could be met via forest residue, mill waste, or non-pine short rotation crops. In short, there are a variety of feasible biomass options and while we are still searching for better ones and trying to improve, we are by no means going to be relegated to draining the forest land in an unsustainable manner. I think education on that sustainability as well as how it can contribute to truly carbon neutral fuels could drastically reduce any societal disapproval that might result from the surface view of logging.

The short answer is of course “It depends.” There are many different things that go into what makes one feedstock better or worse and ‘highest production’ is a tricky thing to pin down. For instance, moisture content differs between biomass types, so whether or not drying is necessary affects efficiency. Additionally, the downstream requirements may further shape which choice would have the highest yield. For example, one feed may be drier or more energy dense, but because it produces a suboptimal ratio of H2/CO or has excessive impurities, the eventual yield is actually worse due to efficiency losses in treatment. Further, not all biomass is easily prepared for gasification. Trees can be ground down to the correct chip size, whereas switchgrass and corn stover might need to be pelletized for gasification.

Auburn’s pilot scale gasifier (~100 lb/hr biomass) is not due to come online until later this year, so I cannot give a definitive “for this system” answer either, but, if pressed, I would lean towards pine as the most viable gasification feed in the southeast due to its availability, H2/CO ratio comparable to most alternatives, energy density, and transportability. It already has a logging and transport infrastructure in place due to the pulp and paper industry, so at least for the near future, its cost from stump to syngas would likely be the most favorable for equivalent quantities of syngas produced.

Could you elaborate on the energetic costs of biomass gasification and explain the gasification process in a little more detail? I understand that the products H2 and CO are relatively high energy for the subsequent Fischer-Tropsch chemistry but have you essentially put the “heat in” at the earlier biomass gasification step?

The gasification process is essentially the partial oxidation of biomass to syngas, which consists of four main steps: drying, pyrolysis, combustion, and reduction. In the drying step, the residual water in the biomass feedstock is vaporized. As such, selecting a biomass source with a low moisture content can reduce energy consumption due to water vaporization. In the pyrolysis step, large organic compounds in the biomass are broken down via complex chemical and physical processes to obtain pyrolysis oil, char, and gases. The combustible products of pyrolysis are then oxidized by either air or pure oxygen to obtain CO2 and H2O in the combustion step. Finally, in the reduction step, the extremely high temperatures in the gasifier cause some of the CO2 and H2O to be cracked, ultimately leading to the formation of CO and H2.

Of the steps described above, drying, pyrolysis, and reduction are all endothermic processes. However, the combustion step is so highly exothermic that it provides all of the thermal energy requirement for the other processes occurring in the gasifier. So in a way, gasification is putting ‘heat in’ to get the high energy density required for the later reaction, but the energy is coming from combustion of a portion of the feed. You are essentially paying some of the input energy to get the remainder into a more useable form.

Has “stump to pump” be quantitatively characterized? One should be able to estimate per acre fuel production, cost, and total energy use going from biomass harvest to final biofuel product. Even a first order estimate would give some idea of the maximum fuel production from a given feed stock, and the areas worth focusing on to improve fuel production.

That is a very to-the-point question. Energy efficiency and costs of the various process steps are among the most crucial aspects of this research. While better answering these questions is still a goal of Auburn’s IGERT program, we can give some rough estimates. In 2011 the U.S. Department of Energy estimated that, with current conversion technologies, about 85 gallons of biofuel could be produced per dry ton of biomass. Managed forests produce on the order of 8 tons of biomass per acre per year, but production rates of up to 14 tons/acre/year are thought to be possible. So, at present, the annual biofuel yield is approximately 680 gallons per acre.

In terms of both cost and energy, the gasification and syngas cleanup steps are among the most expensive parts of the stump to pump process. Biomass gasification products include many contaminants such as H2S, SO2, HCl, and ash, which must be removed in order to prevent catalyst poisoning in the subsequent syngas conversion steps. Of course, the contaminants and quality of the syngas obtained from gasification are dependent on feedstock as well as the operating conditions, so the cleanup cost is highly variable. Additionally, the cost associated with transporting biomass to the gasification/conversion facility is a major economic consideration. A rule of thumb is that outside of about a 50 mile radius, transporting biomass becomes financially infeasible. And while a detailed economic analysis is too complex for us so far, we do have a decent idea of where the largest costs will be and where we can likely make the most impactful improvements.

While our ultimate goal is to make each step in the stump to pump process more efficient, minimizing delivered biomass cost and producing cleaner syngas from gasification are perhaps the most important areas we need to focus on to improve the viability of biofuels as an alternative energy source.

Ísak Jökulsson

Really, any carbonaceous feedstock can be used as the overall process is fairly feedstock agnostic, but I’ve not seen any data on hemp specifically. The gasification step is basically a slightly modified controlled combustion so it is certainly possible that you could get decent H2 and CO yields from hemp or anything else that burns. That said, hemp may go unused because of the regulatory environment in the US, yields per acre, or downstream ease of processing (H2/CO ratio), but again, I’ve seen no data on hemp so I can’t comment on its viability compared to other feeds.

We mostly see woody biomass and switchgrass as the prime ‘plan A’ bioresources for syngas production with other biomass such as agricultural residue or municipal waste as possibilities that are interesting but perhaps less viable on the scale we are interested in.